The major goal of the proposed research is to test the hypothesis that hyperoxia-induced airway remodeling contributes to airway hyperresponsiveness in O2-exposed immature rats, and that prevention of remodeling will inhibit the development of bronchial hyperresponsiveness in these circumstances. Recent studies from the applicant's laboratory have demonstrated that immature rats exposed to 8 days hyperoxia develop both airway remodeling (with thickening of the epithelial and the smooth muscle layers) and airway hyperresponsiveness in vivo and in vitro. Bronchoalveolar lavage (BAL) fluid from these animals promotes DNA synthesis in cultured BALB/3T3 fibroblasts and rat trachea epithelial cells; this mitogenic activity appears to reflect the presence of 1 or more polypeptide growth factors. The simultaneous occurrence of structural (remodeling) and functional (hyperresponsiveness) airway abnormalities in hyperoxia-exposed immature rats affords a unique opportunity to explore the possible casual relationship between these 2 processes, by applying interventions to inhibit the action of growth factors which may contribute to the structural abnormality. Therefore, studies are proposed to: (1) Identify the time course and local mechanisms of hyperoxia-induced increases in airway epithelial mass and smooth muscle layer mass in immature rats. Morphometric analyses of light and electron micrographs of airways from air- and O2-exposed rats, and evaluation of airway cell S- phase traversal in vivo, will test the hypothesis that abnormally increased cellular proliferation contributes to the observed airway thickening. (2) Investigate the expression of polypeptide growth factors in hyperoxia- exposed immature rat lungs, and identify their cellular sources. BA fluid and medium conditioned by bronchoalveolar macrophages or lung fibroblasts from air- and O2-exposed animals will be assayed for mitogenic activities for mesenchymal and epithelial cells, and for the antigenic presence of platelet-derived growth factor and/or insulin-like growth factor-1; in situ appearance of growth factors will also be determined immunohistochemically. (3) Evaluate the potential causal relationship between O2-induced, growth factor-mediated airway remodeling and airway hyperresponsiveness in immature rats. The structure and in vitro responsiveness of bronchial cylinders from 3 airway generations will be correlated. Then 2, interventions will be used to inhibit airway remodeling-administration of heparan sulfate, and administration of blocking anti-growth factor antibodies. Airway morphometry and airway reactivity in vivo will be measured, to test the hypothesis that inhibition of hyperoxia-induced airway remodeling blunts O2-induced airway hyperresponsiveness. From these studies, the investigators hope to learn whether airway remodeling can influence airway responsiveness in animals. Insight gained from this work may shed light on parallel mechanisms that may operate in asthma and in bronchopulmonary dysplasia.